Reworked introduction

I've reworked the introduction of the specification to try and:

* Clarify relationship of DP and SDF
* Clarify exactly how SDF defines a profile of DP
* Bring together all of the requirements around CSV file naming and syntax
* Clarify how to provide a `dialect` (previously this is only obliquely referenced in an example in the DP spec)
Fixed metadata section
Merge pull request #80 from ldodds/gh-pages

Reworked introduction